c: Fix ICE for nested enum redefinitions with/without fixed underlying type [PR112571]

c: Fix ICE for nested enum redefinitions with/without fixed underlying type [PR112571]

Bug 112571 reports an ICE-on-invalid for cases where an enum is
defined, without a fixed underlying type, inside the enum type
specifier for a definition of that same enum with a fixed underlying
type.

The ultimate cause is attempting to access ENUM_UNDERLYING_TYPE in a
case where it is NULL.  Avoid this by clearing
ENUM_FIXED_UNDERLYING_TYPE_P in thie case of inconsistent definitions.

Bootstrapped wth no regressions for x86_64-pc-linux-gnu.

(Note: for this GCC 13 branch backport, the tests were changed to use
-std=c2x not -std=c23, and c23-enum-9.c was changed to expect
different diagnostics because GCC 13 branch doesn't have the C23 tag
compatibility support for redefinitions of tagged types and
enumerators.)

	PR c/112571

gcc/c/
	* c-decl.cc (start_enum): Clear ENUM_FIXED_UNDERLYING_TYPE_P when
	defining without a fixed underlying type an enumeration previously
	declared with a fixed underlying type.

gcc/testsuite/
	* gcc.dg/c23-enum-9.c, gcc.dg/c23-enum-10.c: New tests.

(cherry picked from commit d22d1a9346f27db41459738c6eb404f8f0956e6f)